How do I inject a parent component into a child component?

Question

I'm trying to inject a parent component into a child component. I thought this would be straightforward – simply specify/inject the parent component in the child's constructor():

constructor(private _parent:AppComponent) {}   // child component constructor

I get the following error:

EXCEPTION: Cannot resolve all parameters for ChildComponent(?). Make sure they all have valid type or annotations.

What am I missing?

ChildComponent:

import {Component} from 'angular2/core';
import {AppComponent} from './app.component';

@Component({
  selector: 'child',
  template: `<p>child</p>`
})
export class ChildComponent {
  constructor(private _parent:AppComponent) {}
}

AppComponent:

import {Component} from 'angular2/core';
import {ChildComponent} from './child.component';

@Component({
  selector: 'my-app',
  template: `{{title}} <child></child>
  `,
  directives: [ChildComponent]
})
export class AppComponent {
  title = "Angular 2 - inject parent";
  constructor() { console.clear(); }
}

Plunker

Carton

Mark, the problem is that when you import B from A, and at the same time you import A from B creates a circular dependency (can't go deeper on the subject). Check this plnkr with your case using only one file holding both Child and Parent components. – Adelladella

Thanks @EricMartinez, and thanks for all of your other Angular2 comments and answers as well on SO. I learn a lot from you. – Carton

I want to ammend a word in the above comment. It's actually a circular reference (don't know if it's the same or not, anyway...). I asked about the same thing a time ago in TypeScript chatroom and I got my answer : You can check it here – Adelladella

You're welcome @MarkRajcok and thanks for the words, I really appreciate it, glad to know that I've been helpful. I would really like to ask you as well to add an answer (and if you find more about this subject) explaining this issue. See that there's another user having the same issue. It would be helpful to answer this one and that one. – Adelladella

@EricMartinez, I looked at the other question you referenced. It looks different (and the error is different) -- the template of each component uses the other component. I don't have an answer for that one. (For my case, I only wanted the parent injected so the child could call methods on the parent.) – Carton

Answer 1

See @EricMartinez's comment for the answer. The problem seems to be a circular reference when A imports B and B imports A.

Here's a plunker that uses two files instead of the one file that is in Eric's plunker.

The only change from my original plunker is in the ChildComponent:

import {Component, Inject, forwardRef} from 'angular2/core';
// ....
constructor(@Inject(forwardRef(() => AppComponent)) private _parent:AppComponent)

I don't know for sure if this eliminates the circular reference, since A and B are still importing each other, but it seems to work.

See also https://github.com/angular/angular/issues/3216, where Miško states:

This [not user-friendly declaration using forwardRef()] is a limitation of JS and how the function declarations get hoisted. Whenever you have a circular dependency you will need forwardRef :-( I just don't see a away around it.

I would argue that you should not be in situation where your parent needs to know about the children and children need to know about parent. @Query should take care of most of the use cases.

I am sorry, but while I agree this is a pain in some rare cases, I don't see a way out of it, and hence this issue is not actionable, will close.

Hmm... the reason I tried injecting the parent was because I see two ways for a child to communicate with a parent:

1. the child defines output properties and emits events, which the parent subscribes to
2. the child injects the parent (e.g., Pane might inject Tabs) and can then call methods on the parent

And I was trying to determine when to use each approach. Miško makes it sound like 2. should be rare.

Update: I was thinking about this some more... 1. is better because there is less coupling between the child and the parent. With 1. the child doesn't need to know (and probably shouldn't know) the public API/interface of the parent.
In the reverse direction (e.g., the parent uses @ViewChild (@Query is now deprecated) to get a reference to the child, then calls methods on the child), the coupling is fine, because the parent is using the child component, so it needs to know the public API/interface of the child: i.e., the input and output properties and public methods.

Carton

Yep, you're right, but right now your issue wasn't related to that (maybe using circular dependency was a mistake). Your issue was that you had a circular reference (looks like they're different things). So when you did import {A} from 'B' and import {B} from 'A' you had a circular reference. That's why I removed it in the plnkr. – Adelladella

@EricMartinez, I'm a bit confused. My latest plunk still has a circular reference (right?), but it works. Using @Inject and forwardRef() seems to solve the circular reference problem. Is there more to it than that? – Carton

Oh my god... it works!! I didn't know about that, actually I didn't know that forwardRef could solve this issue using two different files... This is indeed a good finding. I must regret everything I've said so far. Thanks for enlighten me! You're right, so don't be confused at all, I was wrong! – Adelladella

How would you add <parent></parent> to the child components template? – Francie

@BenjaminMcFerren, I wouldn't recommend that, as you'll likely end up in a infinite loop. I believe the only reason a child should (under rare circumstances) get a reference to a parent component would be call public methods/APIs on the parent component. – Carton

ending up in an infinite loop could be true anytime anyone ever uses recursion in general. This feature was possible with Angular1. Is there a way to do this with Angular2? – Francie

@MarkRajcok @Inject(forwardRef(() => AppComponent)) private _parent:AppComponent works but its not injecting the active instance of parent instead its creating a new instance, so how to get the active instance of parent ? – Sultana

@Sultana You can only get the active instance if you omit "@Inject(forwardRef(() => AppComponent))" so you have a constructor like this: constructor(private _parent:AppComponent) – Englishism

A better way is to use a library like ngRx to manage this kinda communication. – Rhythmandblues

Answer 2

you could simply use @Host decorator like this:

import {Component, Host} from 'angular2/core';
// ....
constructor(@Host() private app: AppComponent)

Yelena

I used @Host and it didn't work; was going to down vote your answer, but there might be some cases that it works. So, please explain more why and when it is going to work. – Litho

what exactly is your case? – Yelena

This injects the host component itself not the parent component which renderes the actual component – Piperonal

The case above was to inject specifically the AppComponent @Host will go over the view provider and will inject the closest match, so in this case, we will always get the AppComponent, for your case you could use the @SkipeSelf decorator to skip the component itself – Yelena